1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus which records information on an information recording medium by using a laser beam and, more particularly, to control of a laser beam spot in an optical information recording/reproducing apparatus which uses, as information tracks, both track grooves (guide grooves: grooves) formed in a disk- or card-like recording medium and portions other than the track grooves (portions other than the guide grooves: lands) to record/reproduce information along the information tracks.
2. Related Background Art
In an optical information recording/reproducing apparatus, a light beam emitted from a semiconductor laser is formed into a spot with a diameter of about 1 .mu.m through an objective lens and irradiated on an optical disk. To focus this small light spot on the medium surface of the optical disk, feedback loop control called focusing servo is normally used.
To position the light spot on spiral or concentric information tracks on the medium surface, a tracking servo loop is used. These control loops are normally constituted by the following elements.
1. An optical sensor for receiving a light beam reflected by the disk. PA1 2. An amplifier for amplifying an output from the optical sensor. PA1 3. A control stabilizing compensator for stabilizing an output from the amplifier. PA1 4. An objective lens actuator for focusing the light beam on the disk. PA1 a focus servo for adjusting a focus state of the light beam; PA1 a tracking servo for adjusting a tracking state of the light beam; and PA1 means for checking a parameter of the focus servo and/or the tracking servo in a mode wherein the light beam is tracing the land portion and in a mode wherein the light beam is tracing the groove portion, PA1 wherein the focus servo and/or the tracking servo adjusts the focus state and/or the tracking state of the light beam on the basis of the parameter checked by the check means. PA1 a focus servo for adjusting a focus state of the light beam on the basis of a focus error signal representing the focus state of the light beam; PA1 a tracking servo for adjusting a tracking state of the light beam on the basis of a tracking error signal representing the tracking state of the light beam; and PA1 means for switching an offset signal to be added to the focus error signal and/or the tracking error signal to compensate an offset included in the focus error signal and/or the tracking error signal between a mode wherein the light beam is tracing the land portion and a mode wherein the light beam is tracing the groove portion. PA1 a focus servo for adjusting a focus state of the light beam on the basis of a focus error signal representing the focus state of the light beam; PA1 a tracking servo for adjusting a tracking state of the light beam on the basis of a tracking error signal representing the tracking state of the light beam; and PA1 means for switching a determination level value of the focus error signal and/or the tracking error signal used to detect a deviation of the focus servo and/or the tracking servo between a mode wherein the light beam is tracing the land portion and a mode wherein the light beam is tracing the groove portion. PA1 adjusting a focus state of the light beam by a focus servo; PA1 adjusting a tracking state of the light beam by a tracking servo; and PA1 checking a parameter of the focus servo and/or the tracking servo in a mode wherein the light beam is tracing the land portion and in a mode wherein the light beam is tracing the groove portion, PA1 wherein the focus servo and/or the tracking servo adjusts the focus state and/or the tracking state of the light beam on the basis of the parameter checked in the step of checking the parameter. PA1 adjusting a focus state of the light beam by a focus servo using a focus error signal representing the focus state of the light beam; PA1 adjusting a tracking state of the light beam by a tracking servo using a tracking error signal representing the tracking state of the light beam; and PA1 switching a determination level value of the focus error signal and/or the tracking error signal used to detect a deviation of the focus servo and/or the tracking servo between a mode wherein the light beam is tracing the land portion and a mode wherein the light beam is tracing the groove portion.
A focusing direction position error between the light spot and the optical disk surface is detected on the basis of the amount of light received by the optical sensor, and the objective lens is moved by the actuator, thereby controlling the position of the light spot on the disk surface. Similarly, a tracking direction position error between the track on the disk and the light spot is detected on the basis of the amount of light received by the optical sensor, and the objective lens is moved by the actuator, thereby controlling the position of the light spot on the track.
In recent years, a technique has been used to increase the data memory capacity, in which both guide grooves (grooves) constituting the tracks and portions other than the guide grooves (lands) are used as data tracks. For example, Japanese Laid-Open Patent Application No. 6-176404 discloses an apparatus which records information at both lands and grooves formed in a three-dimensional structure. This apparatus employs a technique of inverting the tracking servo polarity to track the lands and grooves. This prior art also discloses a technique of specifying an arbitrary track by recording address information in the disk as a three-dimensional pattern, i.e., a so-called embossed pit.
This recording technique using both the guide grooves and the portions other than the guide grooves as information tracks to increase the track density is called land/groove recording. In land/groove recording, the track pitch becomes much smaller than that of a conventional technique using only lands or grooves as recording tracks. For example, in a 3.5-inch magnetooptical disk with an ISO standard capacity of 230 MB, the track pitch is about 1.4 .mu.m. To perform land/groove recording at this pitch, a groove structure with a 0.7-.mu.m land width and a 0.7-.mu.m groove width is used. In land/groove recording, therefore, the track pitch is 0.7 .mu.m, i.e., 1/2 that of the 230-MB magnetooptical disk device. For this reason, a highly precise tracking servo is required to realize land/groove recording. In addition, to properly record a signal at such a small track width, or to prevent a signal from being recorded at an adjacent track, the focus servo must be highly precise because an increase in light spot size caused due to defocusing is fatal.
In an actual optical information recording/reproducing apparatus, however, an offset of an electrical circuit, an error in attachment of an optical component, or the like generates an offset in a position error signal such as a focus error signal or a tracking error signal, resulting in a large obstacle against construction of a highly precise servo system.
A small difference between the land width and the groove width cannot be avoided. For this reason, a servo signal, a focus error signal, or a tracking error signal generated upon detecting the light beam reflected by the guide grooves of the disk changes to some extent between the lands and the grooves, so the servo system cannot be constituted by one circuit.
As for a light spot, the difference between a mode wherein the light beam is irradiated on a land and a mode wherein the light beam is irradiated on a groove corresponds to the difference between a mode wherein the light beam is irradiated on the projecting portion of the track and a mode wherein the light beam is irradiated on the recessed portion of the track. For this reason, a focus or tracking error signal changes to some extent. Particularly, when the optical system has an aberration, this difference increases to degrade the imaging performance. Therefore, the characteristic difference between the lands and the grooves poses a serious problem.